Cellulose moulded body and process for its production

ABSTRACT

The invention is concerned with a process for the production of cellulose molded bodies exhibiting high brightness, which is characterized by a combination of the steps of  
     bleaching pulp using a bleaching agent, provided that the bleaching agent does not contain chlorine or any chlorine compound,  
     dissolving the pulp bleached using the bleaching agent in an aqueous tertiary amine-oxide, a moldable cellulose solution being obtained, and  
     processing the moldable cellulose solution into molded bodies.

[0001] The invention is concerned with a cellulose moulded body,particularly a cellulose fibre, and a process for its production.

[0002] For some decades there has been searched for processes for theproduction of cellulose fibres able to substitute the viscose process,today widely employed. As an alternative which is interesting for itsreduced environmental impact among other reasons, it has been found todissolve cellulose without derivatisation in an organic solvent andextrude from this solution moulded bodies, e.g. fibres, films and othermoulded bodies. Fibres thus extruded have received by BISFA (TheInternational Bureau for the Standardization of man made fibers) thegeneric name Lyocell. By an organic solvent, BISFA understands a mixtureof an organic chemical and water.

[0003] It has turned out that as an organic solvent, a mixture of atertiary amine-oxide and water is particularly appropriate for theproduction of cellulose moulded bodies. As the amine-oxide, primarilyN-methylmorpholine-N-oxide (NMMO) is used. Other amine-oxides aredescribed e.g. in EP-A-0 553 070. A process for the production ofmouldable cellulose solutions is known e.g. from EP-A-0 356 419. Theproduction of cellulose moulded bodies using tertiary amine-oxidesgenerally is referred to as amine-oxide process.

[0004] In EP-A-0 356 419, a process for the production of spinnablecellulose solutions is described, wherein as a starting material, amongother substances, a suspension of cellulose in liquid, aqueousN-methylmorpholine-N-oxide (NMMO) is used. This process consists intransforming the suspension in a thin-film treatment apparatus in onesingle step and continuously into a mouldable solution. Finally, themouldable solution is spun into filaments by means of a forming toolsuch as a spinneret, which filaments are conducted across an air gap,wherein they are stretched, into a precipitation bath, wherein thecellulose is precipitated. Afterwards, the cellulose fibres obtained arewashed.

[0005] To avoid an undesired discolouration of the fibres, primarily dueto lignin and coloured organic compounds, it is known to bleach the pulpbefore processing it. For this purpose, raw pulp is reacted with anumber of bleaching agents such as elemental chlorine, chlorinecompounds such as hypochlorite and chlorine dioxide, oxygen, peroxideand ozone, under exactly defined conditions, said reaction being carriedout using a combination of these chemicals in a certain order, which isreferred to as bleaching sequence.

[0006] Classical bleaching processes start using elemental chlorine,which chlorinates or oxidizes to different extents depending on the pHvalue. Hypochlorite and chlorine dioxide oxidize the lignin and colouredcompounds. Between each of the reaction steps of these bleaching agents,usually an alkaline extraction is provided to remove the lignin broughtinto solution and the other compounds from the reaction medium. For anoverview of the technique of pulp bleaching R. P. Singh; The Bleachingof Pulp, TAPPI Press, Atlanta, USA, is cited.

[0007] During pulp bleaching, chlorine-containing bleaching agentsproduce to different extents chlorinated products which are hardlydegradable and for the most part contaminate waste water. Therefore,these processes give rise to environmental concerns, and the use ofchlorine-containing bleaching agents, particularly elemental chlorine,is increasingly restrained. This however implies also a loss ofbleaching quality, since elemental chlorine has a high bleachingcapacity.

[0008] Among the chlorine compounds, chlorine dioxide has a betterbleaching capacity than hypochlorite, but it is more expensive thanhypochlorite, which is more frequently used.

[0009] In the state of the art, bleaching processes which do not involveany chlorine compounds are also employed. For these processes,combinations of oxygen/peroxide and ozone respectively are used. Thepulp thus bleached is referred to as TCF (total chlorine free) pulp,since it is bleached using neither elemental chlorine nor chlorinecompounds. On the other hand, in the literature a pulp which is notbleached using elemental chlorine but chlorine compounds is referred toas ECF (elemental chlorine free) pulp.

[0010] For the purposes of this specification, an ECF pulp bleachedusing hypochlorite is referred to as ECF hypochlorite pulp.

[0011] In the state of the art, the so-called brightness is a measure ofthe intensity of bleaching. It is known from the viscose process thatthere is a relation between the brightness of a bleached pulp and thebrightness of the cellulose products produced from this pulp in such away that pulps having a higher brightness usually may be processed intofibres having also a higher brightness.

[0012] It is the object of the invention to provide a process wherebycellulose moulded bodies may be produced which have a higher brightnessthan cellulose moulded bodies produced from an ECF hypochlorite pulp,while the pulps used have the same starting brightness and whichotherwise the same procedure is employed.

[0013] This object is attained by means of a process for the productionof cellulose moulded bodies which is characterized by a combination ofthe steps of

[0014] bleaching pulp using a bleaching agent, provided that thebleaching agent does not contain chlorine or any chlorine compound,

[0015] dissolving the pulp bleached using the bleaching agent in anaqueous tertiary amine-oxide, a mouldable cellulose solution beingobtained, and

[0016] processing the mouldable cellulose solution into moulded bodies.

[0017] By means of the process according to the invention, it ispossible to particularly produce cellulose fibres having a highbrightness, as well as cellulose films according to the dry/wet-spinningprocess.

[0018] It is surprising to those skilled in the art that TCF pulpsachieve higher fibre brightnesses than ECF hypochlorite pulps when thestarting pulps have the same brightness, with the positive side-effectof being able to produce fibres in a particularly environmentallyfriendly way.

[0019] The invention is also concerned with the bleaching of fibresproduced according to the amine-oxide process using chlorine-freebleaching agents such as oxygen, ozone and particularly hydrogenperoxide.

[0020] A preferred embodiment of the process according to the inventionis characterized in that the fibres or films produced according to theinvention are bleached using a bleaching agent, provided that thebleaching agent does not contain chlorine or any chlorine compound.

[0021] As the tertiary amine-oxide, N-methylmorpholine-N-oxide has shownparticularly good results.

[0022] The invention is also concerned with cellulose moulded bodies,particularly cellulose fibres and films, having a high brightness,produceable according to the process according to the invention.

[0023] Moreover, the invention is concerned with the use of a pulpbleached with a bleaching agent for the production of cellulose mouldedbodies according to the amine-oxide process, provided that the bleachingagent does not contain chlorine or any chlorine compound.

[0024] By means of the following Example and Comparative Example, theinvention will be explained in more detail.

EXAMPLE

[0025] To produce a TCF pulp, pulp (of the beech sulphite pulp type)first was bleached by means of an alkaline oxygen extraction reinforcedby peroxide, afterwards by means of ozone and finally by means ofperoxide, according to known processes. These processes are describedfor instance in R. P. Singh; The Bleaching of Pulp, TAPPI Press,Atlanta, USA, as well as in EP-A-0 426 652.

[0026] The brightness of the TCF pulp obtained, determined according toISO 3688, was 90.6. The pulp had a rapid cuprammonium viscosityaccording to Zellcheming ZM IV/30/62 of 19.8 Pa.s, as well as an αcontent of 90,9.

[0027] Thereafter, this TCF pulp was processed in a known way in astirring vessel into a dope having a composition of 12% of cellulose,77% of NMMO and 11% of water. The viscosity of the dope was 1630 Pa.s(temp.: 90° C., shearing rate: 0.1 s⁻¹).

[0028] The dope was spun into fibres at 120° C. through a spinnerethaving spinning holes exhibiting diameters of 100 μm, according to theknown wet/dry-spinning process (see e.g. EP-A-0 584 318). The brightnessof the fibres produced according to the invention was approximately 50.

[0029] The CIELAB fibre brightness was determined according to thefollowing method: On the fibres, the colour coordinates R_(x), R_(y,)R_(z) were determined according to DIN 6174 and DIN 5033 using the lighttype D65 and at a 10° observation angle. From these colour coordinates,according to DIN 55981 (=chromaticity deviation according toGärtner/Griesser), the CIE brightness and the tint in the red/green-axismay be calculated as follows:

[0030] CIE brightness: W=Y+800*(x0−x)+1700(y0−y)

[0031] Tint: T=900*(x0−x)−650*(y0−y) (Note: T negative=red cast, Tpositive=green cast)

[0032] Y . . . standard chromaticity for green sensitivity (=

[0033] Ry . . . reflectance value green)

[0034] x0, y0 . . . standard chromaticity coordinates of the achromaticpoint (at D65/10° is: x0=0.3138; y0=0.3310)

[0035] x, y . . . standard chromaticity coordinates of the samples; tobe calculated according to:

x=X/(X+Y+Z)

y=Y/(X+Y+Z)

z=1−x−y

[0036] The standard chromaticity numbers X, Y, Z appearing in theseformulae may be calculated from the reflectance values (=filter values)Rx, Ry, Rz at the light type D65 and a 10° observation angle accordingto the formulae:

X=0.94811*Rx

Y=Ry

Z=1.07304*Rz

[0037] Subsequently, these fibres were bleached using hydrogen peroxide(1,5 g of H₂O₂; stabilizing agent: 0,2 g/l of MgSO₄; pH 10,5; fleetratio 1:20; 70° C.; bleaching duration: 3 minutes). The CIELABbrightness of the bleached fibres was 57.

Comparative Example

[0038] To produce an ECF hypochlorite pulp, first the same starting pulpas in the above Example was bleached by means of an alkaline oxygenextraction reinforced by peroxide, afterwards by means of hypochloriteand finally by means of peroxide.

[0039] The brightness of the ECF hypochlorite pulp obtained was 91.6 andthus virtually the same as the one of the TCF pulp produced in theExample.

[0040] Afterwards, a dope and fibres were produced analogously to theabove Example. The fibres produced had a CIELAB brightness of about 39.Thus it becomes clear that the TCF pulp bleached according to theinvention without any chlorine compound may be processed into fibreshaving a higher brightness than the ECF pulp bleached by means of, amongother substances, hypochlorite.

[0041] Subsequently, the fibres produced were bleached as described inthe above Example. The bleached fibres had a CIELAB brightness of about54.

1. A process for the production of cellulose fibres exhibiting highbrightness, characterized by the combination of steps of bleaching pulpusing a bleaching agent, provided that said bleaching agent does notcontain chlorine or any chlorine compound, dissolving said pulp bleachedusing said bleaching agent in an aqueous tertiary amine-oxide, amouldable cellulose solution being obtained, and processing saidmouldable cellulose solution into fibres.
 2. A process according toclaim 1, characterized in that said mouldable cellulose solution isprocessed into fibres according to the dry/wet-spinning process.
 3. Aprocess according to claim 2, characterized in that said fibres arebleached using a bleaching agent, provided that said bleaching agentdoes not contain chlorine or any chlorine compound.
 4. A processaccording to one of the claims 1 or 2, characterized in that as saidtertiary amine-oxide N-methylmorpholine-N-oxide is used.
 5. Use of apulp bleached using a bleaching agent for the production of cellulosemoulded bodies according to the amine-oxide process, provided that saidbleaching agent does not contain chlorine or any chlorine compound.